Definitely no spaces! Aside from the readability problems, spaces in commands is the cause of about 50% of the bugs in DBPro IDEs

Also, always have brackets, like IanM said. (Again, for both readability, and IDE support)
Case sensitivity is a tricky one, but I'd say to go case sensitive, but with autocomplete in the IDE which will find the command regardless of case, like in VC++ (so it actually requires you to press tab to fix the case rather than doing it automatically. Automatically would be very annoying).
It would be nice to get some guidelines for naming conventions, programming style, etc. for user code as well, so that everyone who follows the guidelines will have consistent code.
Finally, this is unrelated, but PLEASE make it so that each source file compiles to its own object file before they are linked! It would improve compile times for large projects dramatically.